In recent years, obtaining accurate information on a time waveform of an optical signal is significantly important in working toward a practical use of information and communication systems utilizing a nonlinear optical effect. Thus, an intensity distribution of the time waveform of an optical signal is measured using an optical sampling oscilloscope, autocorrelator, and so on, in order to obtain accurate information on the time waveform of the optical signal. As a result, amplitude information of the optical signal is obtained from the measured intensity distribution. However, phase information of the optical signal cannot be obtained from the measured intensity distribution. More specifically, information on the time waveform of the optical signal cannot be obtained by merely measuring the intensity distribution of the optical signal.
In view of the above, various methods have been proposed for obtaining a phase of an optical signal (see Non Patent Literatures 1 and 2, for example). The methods disclosed by Non Patent Literatures 1 and 2 are methods for obtaining a phase of an optical signal based on time-resolved spectroscopy. More specifically, the phase of an optical signal is obtained using an ultrafast time gate or a reference light source in the methods according to Non Patent Literatures 1 and 2. Then, the time waveform of the optical signal is reconstructed using the obtained phase.